A vast amount of information has accumulated in recent years regarding the role of calcium (Ca ion 2), as an obligatory mediator of the secretory process. Nevertheless, the mechanism by which Ca ion 2 exerts its critical action is still an enigma. Our approach to elucidating this unknown role of Ca ion 2 involves a study of its interaction with other putative mediators; namely, cyclic AMP, cyclic GMP, and the prostaglandins (PGs); for although there is also a great deal of information concerning the possible functions of these substances in a number of secretory organs, including the adrenal cortex, there is at present an obvious gap in our knowledge of their interactions. Hence, the prime thrust of this investigation is to probe the role of calcium in the mechanism of action of ACTH by a comprehensive study of its interplay with the cyclic nucleotides and the PGs. These studies will be carried out on trypsin-dispersed adrenocortical cells and on isolated cat adrenal glands perfused in situ, and will involve a variety of radio-immunoassays, including PGs, cyclic AMP, cyclic GMP and corticosteroids. The effect of ACTH on cyclic nucleotide and PG metabolism will be determined in the presence and absence of Ca ion 2. Moreover, the effects of exogenous cyclic nucleotide on PG metabolism and the effects of exogenous PGs on cyclic nucleotide metabolism will also be investigated to uncover other potential loci of mediator interaction. It is anticipated that the functional relationships developed as a result of this investigation will help to define the role of Ca ion 2 in ACTH-induced steroid production and release; furthermore it may provide deeper insight into the fundamental mechanisms which control secretory processes, in general.